# A beam of electrons enters a region where a uniform magnetic field acts perpendicular to their velocity. Explain why the electrons follow a circular path and why their speed remains constant throughout this motion.

> IB DP Physics Standard Level (2023 syllabus) — D.3 Motion in electromagnetic fields · Explain · 4 marks

> Electrons in a cathode-ray tube are directed into a region of uniform magnetic field oriented perpendicular to the initial velocity of the electrons.

## Mark scheme (4 marks)

1. The magnetic force on a moving electron is always perpendicular to its velocity.
2. A force that is always perpendicular to the velocity acts as a centripetal force, continuously changing the direction of motion without changing the magnitude of velocity, causing circular motion.
3. Since the magnetic force is always perpendicular to the velocity, it does no work on the electron.
4. Because no work is done, the kinetic energy of the electron remains constant, and therefore its speed remains constant.

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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